This invention relates to printers such as those effecting printing by the thermal transfer of dye materials, and more particularly to an improvement of the paper gripping or clamping mechanism in a printing paper supporting device of printers which includes a drum-shaped platen.
Recently, thermal transfer type printers have come into wide use, in which a thermal head is pressed against a sheet of printing paper supported on a cylindrical platen to transfer a dye material carried on an ink sheet or ribbon to the sheet of paper. Some of these thermal transfer type printers are capable of printing full color images; in such case, a sheet of printing paper must undergo several printing operations in each of which a different primary color is printed thereon so that several primary colors are superposed on each other to produce a full color image. In the case of these full color printers, it is very important that the sheet of paper under printing operations is securely held in position on the platen during printing of several colors so that color dislocations may be minimized. Thus, they usually comprise a paper clamping mechanism for holding the sheet paper securely on the platen drum during printing operations.
A typical paper supporting device of a thermal transfer printer which includes a platen drum for backing up a sheet of printing paper, and a mechanism for clamping it thereon during the printing operation is shown in FIGS. 1 and 2; FIG. 1 shows the state of the device during paper feeding, while FIG. 2 shows it during a printing operation. The printer shown in these figures comprises a thermal head 1, and a take-up roll 2a and a feeding roll 2b for taking up and feeding the ink sheet or ribbon 3 carrying a thermally transferable dye material thereon; the ink sheet 3 is fed from the feeding roll 2b in the direction A to be wound up by the take-up roll 2a. Each sheet of printing paper 6 is guided on a paper guide 5.
The platen drum 4, which rotates in the printing direction shown by the arrow B in FIG. 1, has a recess 4a formed on its peripheral surface. A paper clamper 8, pivoted to the side surfaces of the platen 4, is urged by helical springs 7 toward the main surface of the recess 4a to hold the front end portion of a sheet of printing paper 6 therebetween which is supplied on the guide 5.
On the other hand, a lever 12 is pivotably mounted on the frame of the printing paper. A solenoid 11, when energized, turns the lever 12 in the clockwise direction as viewed in the figure; as a result, the end of the lever 12 abuts and bears against the pin 9 formed on the clamper 8, and lifts the clamper 8 against the urging action of the spring 7, thereby forming a gap between the clamper 8 and the recess 4a on the platen 4. By the way, reference numeral 10 designates a pinch roller 10 rotatably mounted to the frame of the printer to press down the printing paper 6 wrapped around the platen drum 4.
The operation of the device shown in FIGS. 1 and 2 may now be easily understood: When the front end portion of a sheet of printing paper 6 is received between the recess 4a and the clamper 8 which is lifted by means of the energized solenoid 11, the solenoid 11 is de-energized to lower the lever 12. Thus, the clamper 8 is urged down by the spring 7 and the front end portion of the sheet of printing paper 6 is held between the recess 4a and the clamper 8. When the front end portion of the sheet of printing paper 6 is thus held between the recess 4a and the clamper 8, the platen 4 rotates in the direction B, to wrap the paper 6 therearound, so that the tail-end portion of the sheet of paper 6 is pressed down by the pinch roller 10. When the printing paper 6 is thus wrapped around the platen 4, the thermal head 1 is lifted to bear against the platen 4 as shown in FIG. 2, and starts to print images, such as characters and figures, on the sheet of paper 6 by selectively transferring the dye material carried on the ink sheet 3 to the sheet of paper 6.
The above described device, however, suffers from the following disadvantages. Namely, the structure of the clamping device makes it necessary to dispose the solenoid 11 near the pivot of the lever 12. Thus, a large solenoid is needed to overcome the urging action of the spring 7, which, on the other hand, must have a sufficient strength to hold the paper securely between the clamper 8 and the recess 4a during a printing operation; consequently, the printer becomes large-sized and expensive. Further, a large solenoid generates a loud operation noise, which gives rise to an additional problem of the conventional device.